There has been rapid increase of demands for communication services such as a prevalence of communication services, an appearance of various multimedia services and high quality services, and the like. To meet the demands actively, first of all, capacity of communication systems should be increased, and as is more highly required in wireless communications than in wired communications. The reason is that, though available frequency resources are limited and should be shared in wireless communications, the demands for wireless communications are rapidly increasing because of the merits thereof. To increase communication capacity in wireless communication environments, there are two methods: one of them is finding available frequency bands and the other is increasing efficiency of the existing resources. Techniques receiving much attention recently and being developed actively as a method for increasing efficiency of the wireless resources are space-time code techniques, which increase reliability through a diversity gain without increasing bandwidth but by obtaining additional spatial region for utilizing the wireless resources by way of attaching any number of antennas on transmitters/receivers, or increase transmit capacity through parallel transmissions by spatial multiplexing.
Capacity of wireless channels can be increased reasonably by employing MIMO technique. Space-time block code technique is proposed by Alamouti, ‘A simple transmit diversity technique for wireless communications’, IEEE JSAC, vol. 16, no. 8, October 1998, and is a representative transmit diversity technique for overcoming fadings in wireless channels by using any number of antennas in transmitters/receivers. According to this technique, two antennas are used for transmissions and diversity order is the number of transmit antennas multiplied by the number of receive antennas; thus, full-diversity gain can be obtained. However, transmit rate is a value of 1 because only two data signals can be transmitted during two timeslots through two transmit antennas; therefore, spatial multiplexing gain cannot be obtained without regard to the number of receive antennas. Further, transmit techniques for systems having more than three transmit antennas are not presented.
On the other hand, V-BLAST (Vertical Bell Laboratories Layered Space-Time) system proposed by Bell Lab (‘Detection algorithm and initial laboratory results using V-BLAST space-time communication architecture’, IEE, Vol. 35, No. 1, pp. 14˜16, 1999) is a representative technique for obtaining spatial multiplexing gain. In accordance with this technique, in a transmitter, each of the transmit antennas transmits different signals at the same transmit power and rate, and, in a receiver, transmitted signals are detected through three steps and executed thereby: detection ordering; interference nulling; and interference cancellation. Accordingly, unnecessary interferences are removed in sequence, then, SNR (Signal-to-Noise Ratio) can be raised highly. In addition, if the number of the receive antennas is equal to or greater than the number of the transmit antennas, separate data signals corresponding to the number of the transmit antennas can be transmitted at the same time; thus, spatial multiplexing gain can be fully maintained. However, because the number of the receive antennas should be greater than the number of the transmit antennas and diversity order should be maintained a value of 1 to obtain full multiplexing gain, diversity gain cannot be obtained. Therefore, if a signal is restored wrongly in bad channel environments, it affects detecting of a next signal to be transmitted; thus, drastic declines of the performance can be occurred.
Unlike the two foregoing techniques, in recent, there has been published a lot of LD-STC (Linear Dispersion Space-Time Code) using a linear dispersion matrix for FDFR (Full Diversity Full Rate) with optimal diversity-multiplexing tradeoff, which is proposed by Zheng and Tse. An LD-STC is a space-time code for obtaining diversity gain and multiplexing gain at the same time by way of properly combining and transmitting data symbols by using a dispersion matrix. A multiple antenna system using a linear space-time code in a vector-matrix form is represented in Equation 1.
                                          [                                                                                y                    0                                                                                                                    y                    1                                                                                                ⋮                                                                                                  y                                          MT                      -                      1                                                                                            ]                                ︸            Y                          =                                                                                                  [                                                                                                                        [                                                          H                              1                                                        ]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            [                                                          H                              2                                                        ]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                ⋱                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  [                                                          H                              T                                                        ]                                                                                                                ]                                                        ︸                                          H                      stack                                                                      ⁢                                                      [                                                                                                                                                                                                              ⁢                                                          w                                                              1                                ,                                0                                                            1                                                                                                                                                            w                                                          1                              ,                              1                                                        1                                                                                                    …                                                                                                      w                                                          1                              ,                                                              RT                                -                                1                                                                                      1                                                                                                                                                                                                                                                                  ⁢                                                          w                                                              2                                ,                                0                                                            1                                                                                                                                                            w                                                          2                              ,                              1                                                        1                                                                                                    …                                                                                                      w                                                          2                              ,                                                              RT                                -                                1                                                                                      1                                                                                                                                                ⋮                                                                          ⋮                                                                          ⋱                                                                          ⋮                                                                                                                                                                                                                                        ⁢                                                          w                                                              N                                ,                                0                                                            T                                                                                                                                                            w                                                          N                              ,                              1                                                        T                                                                                                    …                                                                                                      w                                                          N                              ,                                                              RT                                -                                1                                                                                      T                                                                                                                ]                                                        ︸                    Φ                                                                              ︸                                  H                  eq                                                      ⁢                                          [                                                                                                    S                        0                                                                                                                                                S                        1                                                                                                                        ⋮                                                                                                                          S                                                  RT                          -                          1                                                                                                                    ]                                            ︸                S                                              +                                    [                                                                                          n                      0                                                                                                                                  n                      1                                                                                                            ⋮                                                                                                              n                                              MT                        -                        1                                                                                                        ]                                      ︸              N                                                          (                  Equation          ⁢                                          ⁢          1                )            
In Equation 1 above, Y is a received signal vector and Hstack shows a multiple antennas channel matrix corresponding to several timeslots. Further, S refers to a transmit signal vector and N represents a Gaussian noise vector. Here, Φ is a linear dispersion matrix of the space-time code; as known from Equation 1, the performance of the system is determined by Φ of the space-time code. Hence, the performance of the system depends on how to design the linear dispersion matrix Φ. There are some conventional linear dispersion space-time codes. As linear dispersion space-time codes optimized for multiple antennas systems having two transmit antennas, there are Tilted-QAM method (‘Structured space-time block codes with optimal diversity-multiplexing tradeoff and minimum delay’, Globecom, in Proc. Vol. 4, pp. 1941-1945, December 2003) proposed by Yao and Wornell, and Golden code (‘The Golden code: a 2×2 full-rate space-time code with non-vanishing determinants’, ISIT 2004, p. 310, July 2004) proposed by Belifiore, Rekaya, and Viterbo. Further, as for linear dispersion space-time codes capable of being designed regardless of the antenna structures, there are TAST code (‘Linear threaded algebraic space-time constellations’, ISIT 2003, pp. 2372-2388, October 2003) proposed by Damen, Gamal, and Beaulieu, and Heath code (‘Capacity maximizing linear space-time code’, IEICE vol. E85-C, no. 3, pp. 428-435, March 2002) proposed by Heath and Paulraj. However, there are some defects in the conventional linear space-time codes. Both Tilted-QAM and Golden codes are optimized only for multiple antennas environments having two transmit/receive antennas. In addition, as for TAST or Heath code, because utilizing and combining complex weights of an identical size cannot maximize coding gain, or generating random matrixes for searching a linear dispersion matrix results in excessively high complexity, looking for a dispersion matrix for an optimized performance is difficult. Hence, to defeat the defects therein, Lee and Oh proposed GOD method (‘Design of space-time codes achieving generalized optimal diversity’, Globecom, in CD, 2005). According to GOD method, because the code can be organized without regard to the antenna structure and spatial multiplexing rate, minimizing the constraints of the complex weights of the linear dispersion matrix raises the coding gain, and optimal linear dispersion matrix can be obtained readily by utilizing the power constraints and the orthogonal constraints; therefore, maximum diversity gain and coding gain can be obtained along with maintaining the multiplexing gain. However, with respect to GOD method, because the optimal complex weights varies according to the antenna structure and multiplexing rate, and the number of the complex weights increases rapidly as the number of the transmit antennas or the spatial multiplexing rate increases, the design complexity increases exponentially as the dimension of the space-time code increases.